1. Field of the Invention
The present invention relates to a hydraulic stepless speed changing device and more particularly to a technique of controlling swash plate angles of movable swash plates of a hydraulic pump and/or a hydraulic motor forming the hydraulic stepless speed changing device.
2. Background Art
Conventionally, there is a known structure of a hydraulic stepless speed changing device (hereafter abbreviated as “HST”) formed of a hydraulic pump and a hydraulic motor, in which a hydraulic servo mechanism controls tilting of movable swash plates of the hydraulic pump and the hydraulic motor to adjust capacities of variable displacement type hydraulic pump and hydraulic motor. As a structure of the hydraulic servo mechanism, a mechanism (automotive control) in which a solenoid valve mounted to the hydraulic pump or the HST automatically tilts the movable swash plates in proportion to an increase of a rotational speed of the hydraulic pump, a mechanism (manual servo control) in which a speed change operation lever mounted to an outer portion of the HST operates the movable swash plate of the hydraulic pump, and the like are known.
On the other hand, the HST having a mechanism (load control mechanism) for carrying out speed control in such manners as to reduce a traveling speed of a work vehicle when a load of an engine is a set value or greater and to increase the traveling speed when the load of the engine is the set value or smaller is known. To put it concretely, the movable swash plate of the HST (hydraulic pump) is shifted to a speed reduction side so that a work load does not exceed a predetermined value to thereby prevent stalling.
As the HST having such hydraulic servo mechanism and load control mechanism, there is an HST as disclosed in Patent Document 1, for example. This HST includes a variable displacement-type hydraulic pump and two hydraulic motors operated by pressurized oil by the hydraulic pump. At least one of the two hydraulic motors is a variable displacement-type hydraulic motor and an output combining mechanism for combining respective output rotations of the two hydraulic motors to take out a single output rotation is provided. With this structure, the output rotation of the variable displacement-type hydraulic motor is adjusted to substantially double output torque to increase the maximum torque in the HST.
The HST disclosed in the Patent Document 1 includes a hydraulic servo mechanism in which a speed changing valve switches a speed changing cylinder interlocked with a movable swash plate to tilt the movable swash plate in the hydraulic pump and movable swash plates of the hydraulic motors can be tilted. With this structure, if load torque on the engine increases during work of the work vehicle at a slushy place or the like, the movable swash plate of the one hydraulic motor formed as the variable displacement-type motor is tilted to thereby increase low-speed torque to perform speed control of the work vehicle.
Patent Document 1: Japanese Unexamined Patent Publication No. 2004-11769
It is true that, with the structure of the HST described in the Patent Document 1, when output rotation of one of the hydraulic motors cannot provide sufficient torque to the work vehicle during traveling of the work vehicle, hydraulic pressure in a main oil path is supplied to or discharged from a switching cylinder to automatically tilt the movable swash plate of the other hydraulic motor. With such a load control mechanism, it is possible to solve the insufficiency of torque to thereby prevent stalling and the like.
However, in the HST disclosed in the Patent Document 1, though the above-described load control mechanism can make up for the insufficiency of torque of the other hydraulic motor by disposing one of the motors as the variable displacement-type hydraulic motor, the movable swash plate of the variable displacement-type hydraulic motor can only be switched by the switching cylinder between a tilted state and a state of zero degree of a swash plate angle. Therefore, accuracy of control of the swash plate angle of the movable swash plate is poor and alignment at a middle angle or the like is impossible. Moreover, it is necessary to dispose at least two hydraulic motors in the main oil path, which complicates the structure and narrows a scope of application. Furthermore, because cradle metal of the hydraulic pump or the like deteriorates and traveling performance changes with the passage of time, it is necessary to make allowances in advance in setting structures of respective members forming the load control mechanism. Moreover, the load control mechanism is for controlling the movable swash plate of the hydraulic motor and cannot be applied to control of the movable swash plate of the hydraulic pump.
It is an object to provide a hydraulic stepless speed changing device for accurately controlling swash plate angles of movable swash plates of a hydraulic pump and/or a hydraulic motor in the HST having a load controlling function.